Лако-красочные материалы — производство

The technology of manufacturing metal pigments goes back to the gold-beaters craft. Upon industrialization, this handcraft was followed by stamping technology, which in some countries and for some special applications is still in use. The stamping technology was followed by ball milling processes, including dry milling (Hametag — process) [5.169] and wet milling (Hall-process) [5.170].

The dry milling process is still the most commonly used method for making gold bronze, whereas the wet milling process is the state-of-the-art technology for manu­facturing aluminum pigments.

Atomized aluminum powder is milled in ball mills in the presence of mineral spirit and a lubricant. After the disintegration process, the pigment slurry is sieved and classified and pressed on a filter press to remove the excess solvent. The press cake (typical solvent content ca. 75 to 80%) is then blended with organic solvents to form an aluminum paste typically containing 65% solids and 35% solvent.

In applications, where aluminum powder is requested (e. g. in powder coatings) or where the rest of the remaining mineral spirit would not be compatible with the application medium (e. g. in printing inks, waterborne coatings and inks, master — batches etc.), the mineral spirit of the press cake is removed in vacuum driers and substituted by any kind of solvent, water, plasticizer, mineral oil or other liquids.

Another very common supplied form of metal effect pigments are the so-called “pellets” or “granulates” [5.171, 5.172]. Here the pigments are supplied in dust-free and easy to handle preparations of metal pigments with a few percent of resin. These pellets are solvent-free and offer the end user a wide variety in formulation besides the other advantages. Both, aluminum and gold bronze pigments are available in pelletized form and are mainly used in printing inks and masterbatches.

A rather new technology is the process for making the so-called “vacuum metallized pigments (VMPs)”, where extremely thin and smooth metallized films are dissolved from the carrier, then disintegrated and processed as a pigment suspension in a variety of solvents [5.173].

Besides the “bare” metal effect pigments there is a whole variety of specially treated or encapsulated metal pigments, to create new effects or to modify the pigment properties according to the needs of special applications:

A CVD-process is known to produce iron-oxide-coated aluminum flakes which form high brilliance colored aluminum pigments with highly interesting interference effects [5.174]. Some of the very new optically variable pigments consist of multilayer flake pigments with a metallic core to obtain metallic reflection and coverage [5.175].

Other metallic pigments are coated with inorganic (mostly silica) or organic (poly­mer) materials to improve their chemical and thermal stability in waterborne coatings, powder coatings, masterbatches, non-conductive coatings etc.

5.3.2.3